setstatetimer.jsx

import React from 'react';
class Timer extends React.Component {
  constructor(props) {
    super(props);
    this.state = {secondsElapsed: 0};
  }
  
  tick: function() {
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});
  }
  
  componentDidMount: function() {
    this.interval = setInterval(this.tick, 1000);
  }

  componentWillUnmount: function() {
    clearInterval(this.interval);
  }

  render: function() {
    return (
      <div>Seconds Elapsed: {this.state.secondsElapsed}</div>
    );
  }
}

export default Timer;